Phosphating a steel strip prior to anealing and temper rolling



United States Patent O 3,288,655 PHOSPHATING A STEEL STRIP PRIOR TO ANNEALING AND TEMPER ROLLING Earnest W. Harwell, Shaker Heights, and Thomas W.

Mastin, Willoughby, Ohio, assignors to The Lubrizol Corporation, Wickliffe, Ohio, a corporation of Ohio No Drawing. Filed Dec. 18, 1963, Ser. No. 331,395

6 Claims. (Cl. 148-615) The invention of this application relates to a process for preparing steel. It relates particularly to such a process which is effective to produce a type of steel which is suitable especially for plating and the manufacture of automobile bumpers, body panels, refrigerator cabinets, etc,, which require a surface relatively free of scratches and like imperfections.

The drawing and deformation of metals such as steel produces extreme pressures and surface temperatures which must, in some way or other, be accommodated if the deformed metal product is to have a surface which is suitable for most purposes; particularly, if the metal prod uct must have a shiny, smooth surface, relatively free from scratches, then these extreme pressures and temperatures must be anticipated and their harmful efiects avoided. One way in which these harmful effects can be avoided is by coating the metal surface, prior to its deformation, with an integral inorganic coating, preferably a phosphate coating, and applying to this phosphated ferrous surface a mixture of borax and soap. This combination of phosphate film, soap, and *borax protects the metal surface during the deforming operation by providing a continuous liquid film between the relatively moving parts throughout a broad range of temperature. That is, the soap is liquid at the lower temperatures, the borax melts and is liquid at an intermediate temperature, and the phosphate film is liquid at a higher temperature. Throughout the entire range of temperature encountered in the deforming operation, therefore, there is a protective liquid film between the relatively moving surfaces of the metal being deformed and the metal of the die and this liquid film is effective to prevent scoring and galling of the metal surfaces.

It is important, in the manufacture of many cold-formed metal products, that the product have a smooth surface. This is especially true where the metal surface must be chromium plated because the chromium will plate much better, and much less chromium will be required, when the metal surface being plated is smooth. The cold-forming operation is unlikely to produce a smooth surface if the raw stock itself is not smooth. Thus, it is important that the metal stock from which a smooth-surfaced product is desired, if itself smooth.

One method by which such smooth surfaced stock can be prepared involves the use of hot-rolled steel strip containing some mill scale. The strip is cut into sheets of appropriate length with respect to the size of the ultimate product desired, then pickled and rinsed. The surface of these pickled sheets is made smooth by belt-sanding and the sheets then are phosphated by immersion in a suitable phosphating bath, rinsed with water, and treated with a soap-borax mixture.

Another method of preparing a smooth-surfaced steel stock involves the use of cold-reduced steel which is then in the full-hard state. Sheets of appropriate size are annealed, cleaned with a mildly alkaline detergent, phosphated as above, rinsed with water, dried, and then passed through a temper mill which imparts a smooth surface to the sheets. The sheets then are treated with the soapborax lubricant. This method has the advantage of not requiring the sanding operation which is both costly and time-consuming.

All previously acceptable methods of preparing smoothsurfaced stock have suffered from the disadvantage of re- 3,288,655 Patented Nov. 29, 1966 quiring the processing of relatively small sheets of steel. This involves a discontinuous operation with all of the inetfi-ciencies which attend such an operation. Quite obviously it would be desirable to subject a continuous strip of steel to the several operations required to produce a smooth-surfaced product, rather than to work with separate, small sheets as has been done heretofore, but a continuous process of this type has itself been subject to many serious disadvantages which have precluded the widespread use of such a process.

For one thing, the uncoiling of annealed steel presents problems. If the strip is simply unwound from the coil, the tendency of the strip to retain its curved shape will cause the surface of the strip to develop cross-breaks as it is uncoiled. These cross-breaks, also referred to as coil-breaks, extend across the width of the strip and appear as creases, and even cracks, in the surface. They extend through the thickness of the strip and thus destroy the smoothness of both sides of the strip. These crossbreaks usually are of sufficient severity that they are not removed by a subsequent temper-rolling operation, nor by the later cold-forming of the metal. They may, in short, persist all the way through to the final product, so that the formation of these cross-breaks is a serious matter which must be avoided.

The formation of these cross-breaks can be avoided by placing the coiled strip under tension so that the strip is not allowed to deviate from a limited path as it is being uncoiled. Thus, it has been proposed to prevent such cross-breaks by mounting the coil on a mandrel, applying under pressure a fairly small diametered working roll to the outer periphery of the coil and in parallelism with the mandrel, and by drawing off the metal strip from the coil along a path which curves snugly around the working roll. Rotation of the coil is resisted by the friction encountered in the rotation of the working roll and by the clamping pressure exerted between the mandrel and the working roll, and in this manner the metal of the strip is cold worked into a state of plasticity thereby preventing the formation of cross breaks. This is an effective means for the elimination of cross-breaks which ordinarily develop as coiled strip is unwound.

Unfortunately, the tension applied to the coiled strip causes the surfaces of the strip in the coil to rub against one another in such a manner as to develop a different type of surface deformity which is referred to as a skid mark. The appearance of skid marks in the surface of strip steel, at this stage of the processing, is not so serious a disadvantage as the appearance of cross-breaks, but is nevertheless a significant factor which militates against the processing of strip steel for the purpose of producing a smooth product. These skid marks are not easily eliminated, either in the subsequent temper-rolling step or the cold-forging step. They usually remain in the final product and thus present a serious disadvantage. For some applications the quality of strip required is such that as much as 40% or more of the strip, as it leaves the temper mill, must be rejected.

It is accordingly, a principal object of this invention to provide a continuous process for the treatment of steel in such a manner as to result in a product suitable for cold-forming and subsequent finishing, most particularly plating.

Another object is to provide a process by which strip steel can be treated in such a manner as to result in a product characterized by the absence of cross-breaks and skid marks.

Another object is to provide a process for the continuous treatment of strip steel in such a manner that the product is suitable for the manufacture of automobile bumpers, refrigerator cabinets and the like.

These and other objects are attainable by the lprocess of preparing steel comprising treating a strip of coldrolled steel in the full-hard state with a phosphating solution to deposit an integral phosphate coating thereon, collecting said phosphated strip as a coil, annealing said coil of phosphated strip in a non-oxidizing atmosphere at a temperature within the range of 900-2400 F., uncoiling said annealed coiled strip while it is under tension, and then temper rolling said strip.

The temper rolled strip which results from the above process is smooth on both sides. It is in finished form, suitable for cutting to length and treatment with a lubricant such as the borax-soap combination referred to earlier. The lubricated sheets can be cold-formed into bumpers or other such finished products. Alternatively, in some cases, the strip can be formed directly into the desired finished product without the necessity of first cutting into sheets. The amount of scrap resulting from the forming operation is considerably reduced in this way. The temper rolled product contains no cross-brakes because the usual means for avoiding such cross-breaks is a part of the process, viz., the tension of the strip as it is uncoiled after having been annealed. Neither are there any skid marks on the product and it appears that the fact that the phosphating step precedes the annealing step is effective to provire a significant degree of protection to preclude the formation of the skid marks which ordinarily result from placing the strip under tension as it is coiled and then uncoiling the strip, and further to prevent sticking of the layers of the coils during the annealing process.

The cold-rolled steel, which may range in thickness from 0.006 to 0.25 inch, is obtained from hot-rolled breakdowns. The cold-rolling step further reduces the thickness of the hot-rolled strip and also imparts some smoothness and develops controlled mechanical properties. The principal function of the cold-rolling, however, is the further reduction in the thickness of the strip. The cold-rolled strip or sheet contemplated for use in the process of this invention is in the full-hard state; it has a hardness value, as measured by the Rockwell Hardness Test, of at least 80.

The cold-reduced strip must be cleaned prior to the step of phosphating. The lubricant used in cold reduction must be removed if a satisfactory phosphate coating is to be achieved. An alkaline detergent solution is effective to remove the residual lubricant film and the most comm-only used detergents for his purpose include caustic soda, sodium orthosilicate and trisodium phosphate. Thorough rinsing of the steel after cleaning is essential to remove all contamination.

' These cleaning and rinsing steps can be carried out most conveniently by spraying. Thus, as the strip leaves the mill the alkaline cleaner can be sprayed onto both surfaces, and then removed by a spray rinse with warm water. In some instances it may be desirable to pass the strip through a tank containing the cleaner, but the 'spray operation is preferred.

The phosphating step The phosphate coating may be applied by any of several well-known methods, including immersion in a phosphating bath, spraying, roller-coating, or flooding the surface of the strip with an aqueous phosphating solution. Of these, immersion is preferred because it affords a more complete contact of the entire area of the strip being treated, with a phosphating solution.

A particularly desirable method of applying the phosphate coating consists of passing the strip through an strips path therethrough. The strip leaves the tank through similarly disposed rubber wipers and then immediately passes through squeegee rolls which remove excess phosphating solution from the strip and cause it to be drained into a lower tank. The contents of this lower tank are circulated back to, the upper tank, and the upper tank in turn contains an overflow weir which drains into the lower tank. The temperature of the phosphating solution is controlled by heating the contents of the lower tank and the solution is replenished, as needed, by adding to the lower tank. The horizontally disposed rubber wipers can be replaced with adjacent rollers through which the strip passes.

This particular method of phosphating, referred to as the tank-over-tank method, allows the strip to travel at a high rate, viz., as required by the cold-rolling operation, and in this respect is superior to other immersion methods of phosphating which require the strip to be dipped into a tank.

As indicated, however, any of the usual methods of phosphating may be employed, within the contemplation of the scope of this invention.

The residence time of the strip in the phosphating bath, i.e., the period of time during which the strip is immersed therein, depends quite obviously on two factors: (1) the speed at which the strip travels through it, and (2) the length of the tank which contains the phosphating bath. In the more ordinary practice of the invention these two factors are coordinated so that this residence time is from about 1 to about 12 seconds. Thus, if the strip travels at a rate of feet per minute and the tank which contains the phosphating solution is 20 feet long, then the residence time is 10 seconds.

The ingredients of the phosphating solution include principally zinc, calcium, phosphate, and nitrate ions. An effective bath can be prepared from a mixture of zinc oxide, phosphoric acid, and calcium nitrate. The proportions of these four ions must be controlled, although within Wide limits, if a suitable protective coating is to be achieved. The zinc ion should be maintained within a range of concentration of from about 1.5 to about 8.0%; the phosphate ion should be maintained within a range of concentration extending from about 3.5 to about 20% the nitrate ion should be maintained within a range of concentration extending from about 5 to about 26% and the calcium ion should be maintained within a range of concentration extending from about 1 to about 4.5%. The total acidity of the solution provides a ready means of ascertaining the overall concentration of the several ions and ordinarily such acidity should be maintained within the range of from about 90 to about 850 points. Preferably the acidity should be maintained within the narrower range of from about to about 300 points. (Total acidity expressed in points is the number of milliliters of 0.1 N base required to titrate a 10-ml. sample of the phosphating solution to the phenolphthalein end point.)

Preferably the components of the phosphating bath should be present in such concentrations as to deposit on the strip from about 50 to about 500 milligrams of an integral phosphate coating per square foot of surface area. Such a bath will have a total acidity within the range of from about to about 225 points and will contain from about 2.7 to about 3.3 percent of zinc ion, from about 7 to about 8 percent of phosphate ion, from about 8 to about 12 percent of nitrate ion, and from about 1.5 to

about 2 percent of calcium ion. In many instances it is desirable that the bath contain also from about 0.6 to

about 1.0 percent of ammonium ion.

In addition to the characterizing zinc, phosphate, nitrate, and calcium ions, it is often desirable to include the ammonium ion and/ or the chloride ion in the aqueous phosphating solutions of this invention. Generally, the ammonium ion will be used in an amount varying from about 0.4 to about 2 percent, preferably from about 0.6

to about 1 percent. Similarly, the chloride ion will be used in an amount varying from about 0.5 to about 3.5 percent, preferably from about 1 to about 1.5 percent.

Specific examples of phosphating solutions which are useful for the purposes of the present invention are shown in Table I, where except for the Points Total Acid, the values given indicate the percentages by weight of the ions in the solution.

ployed, if desired, in the practice of the present invention.

The strip then is coiled and banded. This banded coil must be annealed. This may he a batch operation, as in box annealing, or it may be continuous, as in strand annealing and normalizing. Most annealing operations are of the batch variety where several banded coils are placed in a furnace and the temperature raised gradually to the desired level and maintained there for several TABLE I.PHOSPHA'IING SOLUTION Ion A B C D E F G l H I I i K L M N The phosphating solutions required for the purposes of this invention can be prepared conveniently by first dissolving zinc nitrate, calcium nitrate, and, optionally, ammonium dihydrogen phosphate and zinc chloride, in sufiicient water to yield the required weight percentages of the several ions and then adjusting the acidity of the solution (i.e., the points total acid) by the addition of phosphoric acid and/ or nitric acid.

Thus, it is apparent that the ions of the phosphating solutions used in the practice of this invention may be derived from a variety of compounds and it appears to be of little if any consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts selected to provide the required ions, the resulting solution is effective to serve the purposes of this invention. It is necessary only that these salts or acids be used in amounts to provide the necessary concentration of the required characterizing ions. In addition to the characterizing ions present in the phosphating solution, certain supplementary ions such as bromide, chlorate, perchlorate, nitrite, or perborate ions may be also present to increase the rust inhibiting qualities of the coating, reduce sludging, etc.

The presence of the calcium ion serves to suppress the formation of massive, hydrated crystalline coatings and yield instead a highly .desirable micro-crystalline or amorphous coating. The nitrate ion serves as an oxidizing agent to depolarize the metal surface and increase the coating speed of the phosphating solution. Its presence is likewise essential in the phosphating solutions employed for the purpose of the present invention.

Phosphate coatings which result from the use of the bath described wherein exhibit a scholzite crystal structure and it is believed that this crystal structure contributes significantly to the effectiveness of the phosphate coating in protecting the metal surface during the subsequent operations of the process.

The temperature of the phosphating solution should be maintained, in most cases, at a level within the range of from about 190 to about 240 F. Higher temperatures are useful in some special instances, and in other particular applications it may be desirable to use temperatures ranging down to room temperature.

After the treatment with the phosphating solution the strip must be rinsed again to remove drag-out from the phosphating bath. This is accomplished preferably by means of a spray rinse with cold water. Following this a rinse with dilute chromic acid or borax or some such conventional post-phosphating rinse is advisable to seal the phosphate coating and thus improve its utility as a base for the application of paint, lacquer, varnish, and the like. In lieu of the dilute aqueous chromic acid, dilute aqueous solutions of metal chromates, metal dichromates, chromic acid-phosphoric acid mixtures, and chromic acidmetal dichromate mixtures may be used. Any or all of such known post-phosphating treatments can be emhours. Open-coil annealing is similar except that the coil is not banded during the annealing period. The continuous annealing process requires uncoiling of the banded coil, passing the strip through the furnace, and then recoiling the strip as it emerges from the furnace. This process is much faster. Either the batch or the continuous process may be used in the process of this invention. The advantages noted above do not depend upon the method of annealing.

The temperature to which the strip should be heated in the annealing step ranges from 900 to about 2400 F. In most cases the temperature should be kept between 1800 and 2000 F. It is necessary that the temperature of the strip itself reach these levels; no more is required, but this means that, in the box annealing process, i.e., the bat-ch process, the banded coil must remain in the furnace for several hours because of the time required to reach the annealing temperature as well as the subsequent cooling period.

The atmosphere within the annealing furnace should be inert, and preferably it should be a reducing atmosphere so as to preclude oxidation of the strip surface. A suitable atmosphere consists of the products of partial combustion of a fuel gas in a limited volume of air treated to remove any water vapor, dirt, and carbon. Other inert gaseous mixtures are well known and commonly used.

The next step requires the strip to be placed under tension. This can be accomplished in any of several Ways. One involves placing an expansible mandrel through the center hole of the coil, gripping the transverse edge of the strip in pinch-rolls, applying under pressure a small diametered working roll to the outer periphery of the coil and in parallelism with the mandrel, then drawing off the strip along a path which curves snugly around the working roll. The mandrel is expanded to the extent required to produce the desired tension. As to this latter, the ability of the pinch-rolls to grip the surface of the strip ordinarily will limit the tension under which the strip can be maintained.

The annealed coil, under tension as above, then is uncoiled and fed directly into a temper mill. The temper rolls reduce the strip slightly, up to about 2%, but serve also to impart a smooth finish to the surface and to make it more ductile. The temper-rolled product then is ready for application of the lubricant which is required for the subsequent forming operation. The temper-rolled strip may be cut to the desired length before such application of the lubricant or a lubricant may be applied first, whichever is desired for the particular operation. Strip may also be cold formed without pre-lubricat-ion, in which case the lubricant is applied during the forming operation.

A specific illustration of the overall process is as follows: 24-ga-uge SAE 1010 cold-rolled steel in the fullhard state is passed through an alkaline cleaner spray station to remove the lubricant applied during the cold reducing operation. The strip then is rinsed with warm water (also by spraying), then passed through an immersion tank of the type described previously as a tank-overtank apparatus, containing a phosphating solution having a composition identical with that of solution K; the temperature of the phosphating solution is maintained at 200 F. The size of the tank and the speed of the traveling strip are such that the residence time of the strip in the phosphating bath is 8 seconds. After passing through the phosphating bath the strip is coiled and banded, then placed in a controlled atmosphere annealing furnace whereupon the temperature is increased to 1200 F. to 2000 F. over a period of 8 hours, maintained at that temperature for 2 hours, then allowed to cool over a period of 4 hours.

The annealed coil then is placed on an expansible mandrel from which it is payed olf under tension into the rolls of a temper mill. The tension is such, as explained earlier, as to cause the strip to curve snugly about a portion of the circumference of a small diametered working roll which is applied to the periphery of the coiled strip as it is supported by the expansible mandrel. The tension results from the expansion of the mandrel so as to hold the inner surface of the coil in a fixed position and also by the gripping strength of the pinch rolls through which the strip passes just before it is fed into the temper mill.

The strip as it emerges from the temper mill is suitable immediately for forming operations. Alternatively, it can be cut into sheets which in turn are suitable for forming operations.

If the temper-rolled product is desired in discrete sheets the overall process offers the distinct advantage which results from the fact that the temper-rolled strip of this process can be cut more easily than cold-rolled strip. That is, quite apart from the obvious advantage of being able to carry out the overall process in continuous man-@ ner, there is the added advantage of requiring less expensive cutting equipment to cut the temper-roll strip of this invention into sheets.

Another significant advantage res-ides in the fact that by phosphating the strip after cold-rolling, rather than prior thereto, the phosphate pick-up is considerably reduced. If the phosphate coating is applied to the strip prior to cold-rolling it is necessary to apply a somewhat heavier coating than is desired in the final product because the cold-rolling operation reduces not only the thickness of the strip itself, but also of the phosphate coating which is applied thereto. The problem of pickup is significant and it frequently is necessary to equip the bottom roll of cold-rolling mills and temper-rolling mills with a scraper so that this excess coating can be removed; it also is necessary to shut down a line two or three times a day so that the rolls can be wiped free of this pick-up.

In the process of this invention, however, there is no need to apply any more of the phosphate coating than is required in the ultimate formed product so that the problem of pick-up is minimal.

What is claimed is:

1. The process of preparing steel comprising treating a strip of cold-rolled steel in the full-hard state with a phosphating solution to deposit an integral coating thereon, said phosphating solution containing zinc, phosphate, calcium, and nitrate ions, collecting said phosphated strip as a coil, annealing said coil of phosphate strip in a non-oxidizing atmosphere at a temperature Within the range of 900240-0 F., uncoiling said annealed coiled strip while it is under tension, and then temper rolling said strip.

2. The process of preparing steel comprising treating a strip of cold-rolled steel in the full-hard state with a phosphating solution to deposit an integral coating thereon, said phosphating solution consisting essentially of from about 1.5 to about 8.0% of zinc ion, from about 3.5 to about 20% of phosphate ion, from about 5 to about 26% of nitrate ion, and from about 1 to about 4.5% of calcium ion, collecting said phosphated strip as a coil, annealing said coil of phosphated strip in a non-oxidizing atmosphere at a temperature Within the range of 900 2400 F uncoiling said annealed coiled strip while it is under tension, and then temper rolling said strip.

3. The process of claim 2 wherein the total acidity of the phosphating solution is maintained within the range of from about to about 850 points.

4. The process of preparing steel comprising passing cold-rolled steel in the full-hard state through a phosphating solution to deposit an integral phosphate coating thereon, said phosphating solution consisting essentially of from about 1.5 to about 8.0% of zinc ion, from about 3.5 to about 20% of phosphate ion, from about 5 to about 26% of nitrate ion, and from about 1 to about 4.5% of calcium ion, and having a total acidity within the range of from about 90 to about 850 points, collecting said phosphated strip as a coil, annealing said coil of phosphated strip in a non-oxidizing atmosphere at a temperature within the range of 9002400 F., uncoiling said annealed coiled strip while it is under tension, and then temper rolling said strip.

5. The process of claim 4 wherein the integral phosphate coating exhibits a substantially scholzite crystal structure.

6. The process of claim 4 wherein the density of the integral phosphate coating is within the range of from about 50 to about 500 milligrams per square foot of surface area.

References Cited by the Examiner UNITED STATES PATENTS 2,310,451 2/ 1943 Marshall 1486. 15 2,501,846 3/1950 Gifford 148-615 3,007,817 11/1961 Cavanagh et al. 148--6.15 3,090,709 5/1963 Hendricks 117127 3,104,177 9/1963 Goldsmith 1486.l5 3,114,661 12/1963 Palm 148-6.15

ALFRED L. LEAVITT, Primary Examiner.

DAVID L. RECK, Examiner.

H. F. SAITO, R. S, KENDALL, Assistant Examiners. 

1. THE PROCESS OF PREPARING STEEL COMPRISING TREATING A STRIP OF COLD-ROLLED STEEL IN THE FULL-HARD STATE WITH A PHOSPHATING SOLUTION TO DEPOSIT AN INTEGRAL COATING THEREON, SAID PHOSPHATING SOLUTION CONTAINING ZINC, PHOSPHATE, CALCIUM, AND NITRATE IONS, COLLECTING SAID PHOSPHATED STRIP AS A COIL, ANNEALING SAID COIL OF PHOSPHATE STRIP IN A NON-OXIDIZING ATMOSPHERE AT A TEMPERATURE WITHIN THE RANGE OF 900-2400*F., UNCOILING SAID ANNEALED COILED STRIP WHILE IT IS UNDER TENSION, AND THEN TEMPER ROLLING SAID STRIP. 